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><H1
CLASS="SECT1"
><A
NAME="GIST-IMPLEMENTATION"
>53.3. Implementation</A
></H1
><P
>   There are seven methods that an index operator class for
   <ACRONYM
CLASS="ACRONYM"
>GiST</ACRONYM
> must provide, and an eighth that is optional.
   Correctness of the index is ensured
   by proper implementation of the <CODE
CLASS="FUNCTION"
>same</CODE
>, <CODE
CLASS="FUNCTION"
>consistent</CODE
>
   and <CODE
CLASS="FUNCTION"
>union</CODE
> methods, while efficiency (size and speed) of the
   index will depend on the <CODE
CLASS="FUNCTION"
>penalty</CODE
> and <CODE
CLASS="FUNCTION"
>picksplit</CODE
>
   methods.
   The remaining two basic methods are <CODE
CLASS="FUNCTION"
>compress</CODE
> and
   <CODE
CLASS="FUNCTION"
>decompress</CODE
>, which allow an index to have internal tree data of
   a different type than the data it indexes. The leaves are to be of the
   indexed data type, while the other tree nodes can be of any C struct (but
   you still have to follow <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> data type rules here,
   see about <TT
CLASS="LITERAL"
>varlena</TT
> for variable sized data). If the tree's
   internal data type exists at the SQL level, the <TT
CLASS="LITERAL"
>STORAGE</TT
> option
   of the <TT
CLASS="COMMAND"
>CREATE OPERATOR CLASS</TT
> command can be used.
   The optional eighth method is <CODE
CLASS="FUNCTION"
>distance</CODE
>, which is needed
   if the operator class wishes to support ordered scans (nearest-neighbor
   searches).
 </P
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><CODE
CLASS="FUNCTION"
>consistent</CODE
></DT
><DD
><P
>       Given an index entry <TT
CLASS="LITERAL"
>p</TT
> and a query value <TT
CLASS="LITERAL"
>q</TT
>,
       this function determines whether the index entry is
       <SPAN
CLASS="QUOTE"
>"consistent"</SPAN
> with the query; that is, could the predicate
       <SPAN
CLASS="QUOTE"
>"<TT
CLASS="REPLACEABLE"
><I
>indexed_column</I
></TT
>
       <TT
CLASS="REPLACEABLE"
><I
>indexable_operator</I
></TT
> <TT
CLASS="LITERAL"
>q</TT
>"</SPAN
> be true for
       any row represented by the index entry?  For a leaf index entry this is
       equivalent to testing the indexable condition, while for an internal
       tree node this determines whether it is necessary to scan the subtree
       of the index represented by the tree node.  When the result is
       <TT
CLASS="LITERAL"
>true</TT
>, a <TT
CLASS="LITERAL"
>recheck</TT
> flag must also be returned.
       This indicates whether the predicate is certainly true or only possibly
       true.  If <TT
CLASS="LITERAL"
>recheck</TT
> = <TT
CLASS="LITERAL"
>false</TT
> then the index has
       tested the predicate condition exactly, whereas if <TT
CLASS="LITERAL"
>recheck</TT
>
       = <TT
CLASS="LITERAL"
>true</TT
> the row is only a candidate match.  In that case the
       system will automatically evaluate the
       <TT
CLASS="REPLACEABLE"
><I
>indexable_operator</I
></TT
> against the actual row value to see
       if it is really a match.  This convention allows
       <ACRONYM
CLASS="ACRONYM"
>GiST</ACRONYM
> to support both lossless and lossy index
       structures.
      </P
><P
>        The <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> declaration of the function must look like this:

</P><PRE
CLASS="PROGRAMLISTING"
>CREATE OR REPLACE FUNCTION my_consistent(internal, data_type, smallint, oid, internal)
RETURNS bool
AS 'MODULE_PATHNAME'
LANGUAGE C STRICT;</PRE
><P>

        And the matching code in the C module could then follow this skeleton:

</P><PRE
CLASS="PROGRAMLISTING"
>Datum       my_consistent(PG_FUNCTION_ARGS);
PG_FUNCTION_INFO_V1(my_consistent);

Datum
my_consistent(PG_FUNCTION_ARGS)
{
    GISTENTRY  *entry = (GISTENTRY *) PG_GETARG_POINTER(0);
    data_type  *query = PG_GETARG_DATA_TYPE_P(1);
    StrategyNumber strategy = (StrategyNumber) PG_GETARG_UINT16(2);
    /* Oid subtype = PG_GETARG_OID(3); */
    bool       *recheck = (bool *) PG_GETARG_POINTER(4);
    data_type  *key = DatumGetDataType(entry-&gt;key);
    bool        retval;

    /*
     * determine return value as a function of strategy, key and query.
     *
     * Use GIST_LEAF(entry) to know where you're called in the index tree,
     * which comes handy when supporting the = operator for example (you could
     * check for non empty union() in non-leaf nodes and equality in leaf
     * nodes).
     */

    *recheck = true;        /* or false if check is exact */

    PG_RETURN_BOOL(retval);
}</PRE
><P>

       Here, <TT
CLASS="VARNAME"
>key</TT
> is an element in the index and <TT
CLASS="VARNAME"
>query</TT
>
       the value being looked up in the index. The <TT
CLASS="LITERAL"
>StrategyNumber</TT
>
       parameter indicates which operator of your operator class is being
       applied &mdash; it matches one of the operator numbers in the
       <TT
CLASS="COMMAND"
>CREATE OPERATOR CLASS</TT
> command.  Depending on what operators
       you have included in the class, the data type of <TT
CLASS="VARNAME"
>query</TT
> could
       vary with the operator, but the above skeleton assumes it doesn't.
      </P
></DD
><DT
><CODE
CLASS="FUNCTION"
>union</CODE
></DT
><DD
><P
>       This method consolidates information in the tree.  Given a set of
       entries, this function generates a new index entry that represents
       all the given entries.
      </P
><P
>        The <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> declaration of the function must look like this:

</P><PRE
CLASS="PROGRAMLISTING"
>CREATE OR REPLACE FUNCTION my_union(internal, internal)
RETURNS internal
AS 'MODULE_PATHNAME'
LANGUAGE C STRICT;</PRE
><P>

        And the matching code in the C module could then follow this skeleton:

</P><PRE
CLASS="PROGRAMLISTING"
>Datum       my_union(PG_FUNCTION_ARGS);
PG_FUNCTION_INFO_V1(my_union);

Datum
my_union(PG_FUNCTION_ARGS)
{
    GistEntryVector *entryvec = (GistEntryVector *) PG_GETARG_POINTER(0);
    GISTENTRY  *ent = entryvec-&gt;vector;
    data_type  *out,
               *tmp,
               *old;
    int         numranges,
                i = 0;

    numranges = entryvec-&gt;n;
    tmp = DatumGetDataType(ent[0].key);
    out = tmp;

    if (numranges == 1)
    {
        out = data_type_deep_copy(tmp);

        PG_RETURN_DATA_TYPE_P(out);
    }

    for (i = 1; i &lt; numranges; i++)
    {
        old = out;
        tmp = DatumGetDataType(ent[i].key);
        out = my_union_implementation(out, tmp);
    }

    PG_RETURN_DATA_TYPE_P(out);
}</PRE
><P>
      </P
><P
>        As you can see, in this skeleton we're dealing with a data type
        where <TT
CLASS="LITERAL"
>union(X, Y, Z) = union(union(X, Y), Z)</TT
>. It's easy
        enough to support data types where this is not the case, by
        implementing the proper union algorithm in this
        <ACRONYM
CLASS="ACRONYM"
>GiST</ACRONYM
> support method.
      </P
><P
>        The <CODE
CLASS="FUNCTION"
>union</CODE
> implementation function should return a
        pointer to newly <CODE
CLASS="FUNCTION"
>palloc()</CODE
>ed memory. You can't just
        return whatever the input is.
      </P
></DD
><DT
><CODE
CLASS="FUNCTION"
>compress</CODE
></DT
><DD
><P
>       Converts the data item into a format suitable for physical storage in
       an index page.
      </P
><P
>        The <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> declaration of the function must look like this:

</P><PRE
CLASS="PROGRAMLISTING"
>CREATE OR REPLACE FUNCTION my_compress(internal)
RETURNS internal
AS 'MODULE_PATHNAME'
LANGUAGE C STRICT;</PRE
><P>

        And the matching code in the C module could then follow this skeleton:

</P><PRE
CLASS="PROGRAMLISTING"
>Datum       my_compress(PG_FUNCTION_ARGS);
PG_FUNCTION_INFO_V1(my_compress);

Datum
my_compress(PG_FUNCTION_ARGS)
{
    GISTENTRY  *entry = (GISTENTRY *) PG_GETARG_POINTER(0);
    GISTENTRY  *retval;

    if (entry-&gt;leafkey)
    {
        /* replace entry-&gt;key with a compressed version */
        compressed_data_type *compressed_data = palloc(sizeof(compressed_data_type));

        /* fill *compressed_data from entry-&gt;key ... */

        retval = palloc(sizeof(GISTENTRY));
        gistentryinit(*retval, PointerGetDatum(compressed_data),
                      entry-&gt;rel, entry-&gt;page, entry-&gt;offset, FALSE);
    }
    else
    {
        /* typically we needn't do anything with non-leaf entries */
        retval = entry;
    }

    PG_RETURN_POINTER(retval);
}</PRE
><P>
      </P
><P
>       You have to adapt <TT
CLASS="REPLACEABLE"
><I
>compressed_data_type</I
></TT
> to the specific
       type you're converting to in order to compress your leaf nodes, of
       course.
      </P
><P
>        Depending on your needs, you could also need to care about
        compressing <TT
CLASS="LITERAL"
>NULL</TT
> values in there, storing for example
        <TT
CLASS="LITERAL"
>(Datum) 0</TT
> like <TT
CLASS="LITERAL"
>gist_circle_compress</TT
> does.
      </P
></DD
><DT
><CODE
CLASS="FUNCTION"
>decompress</CODE
></DT
><DD
><P
>       The reverse of the <CODE
CLASS="FUNCTION"
>compress</CODE
> method.  Converts the
       index representation of the data item into a format that can be
       manipulated by the database.
      </P
><P
>        The <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> declaration of the function must look like this:

</P><PRE
CLASS="PROGRAMLISTING"
>CREATE OR REPLACE FUNCTION my_decompress(internal)
RETURNS internal
AS 'MODULE_PATHNAME'
LANGUAGE C STRICT;</PRE
><P>

        And the matching code in the C module could then follow this skeleton:

</P><PRE
CLASS="PROGRAMLISTING"
>Datum       my_decompress(PG_FUNCTION_ARGS);
PG_FUNCTION_INFO_V1(my_decompress);

Datum
my_decompress(PG_FUNCTION_ARGS)
{
    PG_RETURN_POINTER(PG_GETARG_POINTER(0));
}</PRE
><P>

        The above skeleton is suitable for the case where no decompression
        is needed.
      </P
></DD
><DT
><CODE
CLASS="FUNCTION"
>penalty</CODE
></DT
><DD
><P
>       Returns a value indicating the <SPAN
CLASS="QUOTE"
>"cost"</SPAN
> of inserting the new
       entry into a particular branch of the tree.  Items will be inserted
       down the path of least <CODE
CLASS="FUNCTION"
>penalty</CODE
> in the tree.
       Values returned by <CODE
CLASS="FUNCTION"
>penalty</CODE
> should be non-negative.
       If a negative value is returned, it will be treated as zero.
      </P
><P
>        The <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> declaration of the function must look like this:

</P><PRE
CLASS="PROGRAMLISTING"
>CREATE OR REPLACE FUNCTION my_penalty(internal, internal, internal)
RETURNS internal
AS 'MODULE_PATHNAME'
LANGUAGE C STRICT;  -- in some cases penalty functions need not be strict</PRE
><P>

        And the matching code in the C module could then follow this skeleton:

</P><PRE
CLASS="PROGRAMLISTING"
>Datum       my_penalty(PG_FUNCTION_ARGS);
PG_FUNCTION_INFO_V1(my_penalty);

Datum
my_penalty(PG_FUNCTION_ARGS)
{
    GISTENTRY  *origentry = (GISTENTRY *) PG_GETARG_POINTER(0);
    GISTENTRY  *newentry = (GISTENTRY *) PG_GETARG_POINTER(1);
    float      *penalty = (float *) PG_GETARG_POINTER(2);
    data_type  *orig = DatumGetDataType(origentry-&gt;key);
    data_type  *new = DatumGetDataType(newentry-&gt;key);

    *penalty = my_penalty_implementation(orig, new);
    PG_RETURN_POINTER(penalty);
}</PRE
><P>
      </P
><P
>        The <CODE
CLASS="FUNCTION"
>penalty</CODE
> function is crucial to good performance of
        the index. It'll get used at insertion time to determine which branch
        to follow when choosing where to add the new entry in the tree. At
        query time, the more balanced the index, the quicker the lookup.
      </P
></DD
><DT
><CODE
CLASS="FUNCTION"
>picksplit</CODE
></DT
><DD
><P
>       When an index page split is necessary, this function decides which
       entries on the page are to stay on the old page, and which are to move
       to the new page.
      </P
><P
>        The <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> declaration of the function must look like this:

</P><PRE
CLASS="PROGRAMLISTING"
>CREATE OR REPLACE FUNCTION my_picksplit(internal, internal)
RETURNS internal
AS 'MODULE_PATHNAME'
LANGUAGE C STRICT;</PRE
><P>

        And the matching code in the C module could then follow this skeleton:

</P><PRE
CLASS="PROGRAMLISTING"
>Datum       my_picksplit(PG_FUNCTION_ARGS);
PG_FUNCTION_INFO_V1(my_picksplit);

Datum
my_picksplit(PG_FUNCTION_ARGS)
{
    GistEntryVector *entryvec = (GistEntryVector *) PG_GETARG_POINTER(0);
    OffsetNumber maxoff = entryvec-&gt;n - 1;
    GISTENTRY  *ent = entryvec-&gt;vector;
    GIST_SPLITVEC *v = (GIST_SPLITVEC *) PG_GETARG_POINTER(1);
    int         i,
                nbytes;
    OffsetNumber *left,
               *right;
    data_type  *tmp_union;
    data_type  *unionL;
    data_type  *unionR;
    GISTENTRY **raw_entryvec;

    maxoff = entryvec-&gt;n - 1;
    nbytes = (maxoff + 1) * sizeof(OffsetNumber);

    v-&gt;spl_left = (OffsetNumber *) palloc(nbytes);
    left = v-&gt;spl_left;
    v-&gt;spl_nleft = 0;

    v-&gt;spl_right = (OffsetNumber *) palloc(nbytes);
    right = v-&gt;spl_right;
    v-&gt;spl_nright = 0;

    unionL = NULL;
    unionR = NULL;

    /* Initialize the raw entry vector. */
    raw_entryvec = (GISTENTRY **) malloc(entryvec-&gt;n * sizeof(void *));
    for (i = FirstOffsetNumber; i &lt;= maxoff; i = OffsetNumberNext(i))
        raw_entryvec[i] = &amp;(entryvec-&gt;vector[i]);

    for (i = FirstOffsetNumber; i &lt;= maxoff; i = OffsetNumberNext(i))
    {
        int         real_index = raw_entryvec[i] - entryvec-&gt;vector;

        tmp_union = DatumGetDataType(entryvec-&gt;vector[real_index].key);
        Assert(tmp_union != NULL);

        /*
         * Choose where to put the index entries and update unionL and unionR
         * accordingly. Append the entries to either v_spl_left or
         * v_spl_right, and care about the counters.
         */

        if (my_choice_is_left(unionL, curl, unionR, curr))
        {
            if (unionL == NULL)
                unionL = tmp_union;
            else
                unionL = my_union_implementation(unionL, tmp_union);

            *left = real_index;
            ++left;
            ++(v-&gt;spl_nleft);
        }
        else
        {
            /*
             * Same on the right
             */
        }
    }

    v-&gt;spl_ldatum = DataTypeGetDatum(unionL);
    v-&gt;spl_rdatum = DataTypeGetDatum(unionR);
    PG_RETURN_POINTER(v);
}</PRE
><P>
      </P
><P
>        Like <CODE
CLASS="FUNCTION"
>penalty</CODE
>, the <CODE
CLASS="FUNCTION"
>picksplit</CODE
> function
        is crucial to good performance of the index.  Designing suitable
        <CODE
CLASS="FUNCTION"
>penalty</CODE
> and <CODE
CLASS="FUNCTION"
>picksplit</CODE
> implementations
        is where the challenge of implementing well-performing
        <ACRONYM
CLASS="ACRONYM"
>GiST</ACRONYM
> indexes lies.
      </P
></DD
><DT
><CODE
CLASS="FUNCTION"
>same</CODE
></DT
><DD
><P
>       Returns true if two index entries are identical, false otherwise.
      </P
><P
>        The <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> declaration of the function must look like this:

</P><PRE
CLASS="PROGRAMLISTING"
>CREATE OR REPLACE FUNCTION my_same(internal, internal, internal)
RETURNS internal
AS 'MODULE_PATHNAME'
LANGUAGE C STRICT;</PRE
><P>

        And the matching code in the C module could then follow this skeleton:

</P><PRE
CLASS="PROGRAMLISTING"
>Datum       my_same(PG_FUNCTION_ARGS);
PG_FUNCTION_INFO_V1(my_same);

Datum
my_same(PG_FUNCTION_ARGS)
{
    prefix_range *v1 = PG_GETARG_PREFIX_RANGE_P(0);
    prefix_range *v2 = PG_GETARG_PREFIX_RANGE_P(1);
    bool       *result = (bool *) PG_GETARG_POINTER(2);

    *result = my_eq(v1, v2);
    PG_RETURN_POINTER(result);
}</PRE
><P>

        For historical reasons, the <CODE
CLASS="FUNCTION"
>same</CODE
> function doesn't
        just return a Boolean result; instead it has to store the flag
        at the location indicated by the third argument.
      </P
></DD
><DT
><CODE
CLASS="FUNCTION"
>distance</CODE
></DT
><DD
><P
>       Given an index entry <TT
CLASS="LITERAL"
>p</TT
> and a query value <TT
CLASS="LITERAL"
>q</TT
>,
       this function determines the index entry's
       <SPAN
CLASS="QUOTE"
>"distance"</SPAN
> from the query value.  This function must be
       supplied if the operator class contains any ordering operators.
       A query using the ordering operator will be implemented by returning
       index entries with the smallest <SPAN
CLASS="QUOTE"
>"distance"</SPAN
> values first,
       so the results must be consistent with the operator's semantics.
       For a leaf index entry the result just represents the distance to
       the index entry; for an internal tree node, the result must be the
       smallest distance that any child entry could have.
      </P
><P
>        The <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> declaration of the function must look like this:

</P><PRE
CLASS="PROGRAMLISTING"
>CREATE OR REPLACE FUNCTION my_distance(internal, data_type, smallint, oid)
RETURNS float8
AS 'MODULE_PATHNAME'
LANGUAGE C STRICT;</PRE
><P>

        And the matching code in the C module could then follow this skeleton:

</P><PRE
CLASS="PROGRAMLISTING"
>Datum       my_distance(PG_FUNCTION_ARGS);
PG_FUNCTION_INFO_V1(my_distance);

Datum
my_distance(PG_FUNCTION_ARGS)
{
    GISTENTRY  *entry = (GISTENTRY *) PG_GETARG_POINTER(0);
    data_type  *query = PG_GETARG_DATA_TYPE_P(1);
    StrategyNumber strategy = (StrategyNumber) PG_GETARG_UINT16(2);
    /* Oid subtype = PG_GETARG_OID(3); */
    data_type  *key = DatumGetDataType(entry-&gt;key);
    double      retval;

    /*
     * determine return value as a function of strategy, key and query.
     */

    PG_RETURN_FLOAT8(retval);
}</PRE
><P>

       The arguments to the <CODE
CLASS="FUNCTION"
>distance</CODE
> function are identical to
       the arguments of the <CODE
CLASS="FUNCTION"
>consistent</CODE
> function, except that no
       recheck flag is used.  The distance to a leaf index entry must always
       be determined exactly, since there is no way to re-order the tuples
       once they are returned.  Some approximation is allowed when determining
       the distance to an internal tree node, so long as the result is never
       greater than any child's actual distance.  Thus, for example, distance
       to a bounding box is usually sufficient in geometric applications.  The
       result value can be any finite <TT
CLASS="TYPE"
>float8</TT
> value.  (Infinity and
       minus infinity are used internally to handle cases such as nulls, so it
       is not recommended that <CODE
CLASS="FUNCTION"
>distance</CODE
> functions return these
       values.)
      </P
></DD
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